1. Field of the Invention
This invention relates to a shoe clearance over-adjustment prevention apparatus for a drum brake which ceases automatic adjustment of a shoe clearance between brake shoes and a brake drum (hereinafter xe2x80x9cshoe-to-drum clearancexe2x80x9d) when a brake temperature exceeds a predetermined value or range.
2. Description of the Related Arts
Generally, a drum brake has an automatic shoe clearance adjustment device maintaining a constant shoe-to-drum clearance depending on the amount of lining wear on the brake shoe.
If the drum brake is repeatedly applied in a case for example on a long slope, a brake drum temperature increases and the material expands to create an apparent shoe-to-drum clearance expansion, i.e., temporary expansion of the shoe-to-drum clearance to the extent of brake drum expansion.
Even if the apparent shoe-to-drum clearance expansion progresses, an action of the automatic shoe clearance adjustment device is continued. Therefore, when a brake drum diameter becomes smaller due to the temperature fall, the automatic shoe clearance adjustment device causes a problem of dragging because of an overly diminished shoe-to-drum clearance.
If a shoe-to-drum clearance in a normal use (in a use under a normal temperature) is preset to be larger for the purpose of avoiding the above-mentioned dragging, the shoe clearance over-adjustment may be prevented and on the other hand a stroking, such as a brake pedal stroke and a brake lever stroke, becomes longer causing delay in a brake effect and a disconcerting brake feeling.
As a preventive technology against the above problems, for example, the Japanese Patent Publication No. 58-49739 discloses a shoe clearance over-adjustment prevention apparatus.
The shoe clearance over-adjustment prevention apparatus as disclosed in the prior art is explained with reference to a plan view of a leading trailing (LT) type drum brake having a parking brake mechanism and an incremental type automatic shoe clearance adjustment device as shown in FIG. 9.
A strut 21, a part of which composes a screw mechanism, is extended between a left brake shoe 11 and a brake lever 30 pivotally supported on a right brake shoe 12 and is disposed adjacent to a wheel cylinder 40 for a service brake, which moves a pair of brake shoes 11, 12 to separate upper adjacent ends of the brake shoes 11, 12 apart from each other, thereby restricting a returning (initial) position of the pair of brake shoes 11, 12 with an abutment point with an anchor 15 as the fulcrum.
The strut 21 comprises a bolt 22 and a socket 23, both axially slidably fitting each other, and automatically adjust the shoe-to-drum clearance by moving to follow the right brake shoe 12 by an adjustment lever 50 receiving a force of an adjustment spring 19 in response to the over-movement of the pair of brake shoes 11, 12 during the operation of the wheel cylinder 40 and rotating a nut 24 screwing on the bolt 22 to project the bolt 22 fitting in the socket 23 to extend an overall length of the strut 21.
The conventional shoe clearance over-adjustment prevention apparatus includes a bimetal element 13, a superposed section thereof being sandwiched between the socket 23 and the nut 24, and the bolt 22 penetrating the superposed section as shown in an enlarged view of FIG. 10(A). The shoe clearance over-adjustment prevention apparatus is structured to prevent over-adjustment by ceasing an automatic shoe clearance adjustment by opening the superposed section of the bimetal element 13 against the force of the adjustment spring 19, as shown in FIG. 10(B), when the brake temperature reaches to a predetermined value or range.
The above-described conventional automatic shoe clearance over-adjustment prevention apparatus has the following drawbacks:
A use of the conventional type of shoe clearance over-adjustment prevention apparatus, in which the bimetal element is sandwiched to be installed between the nut 22 and the socket 23 both of which may axially move apart from each other, is limited to its application only in an automatic shoe clearance adjustment device for an incremental type and therefore is not extended to be applied to a one shot type, which provides a poor applicability.
A width of an engagement groove of the strut 21 at the left side in an axial direction in FIG. 9 relative to an engagement section of the left brake shoe 11 and a width of an engagement groove of the strut 21 at the right side relative to an engagement section of the brake lever 30, are narrow. Therefore, a play in a rotational direction relative to an axis of the strut 21 is created.
The strut 21 rotates corresponding to the play without the shoe clearance adjustment during the automatic shoe clearance adjustment, which degrades the automatic shoe clearance adjustment function requiring a fine tuning.
This invention was made to remove the aforementioned drawbacks, and an object of this invention is to provide an automatic shoe clearance over-adjustment prevention apparatus being applicable not only to a drum brake with an incremental type automatic shoe clearance adjustment device but also to a drum brake with the one shot type automatic shoe clearance adjustment device, thereby increasing the applicability and enabling to utilize common thermo-sensitive members.
Another object of this invention is to provide a shoe clearance over-adjustment prevention apparatus which lessens the effect of play and maintains a stable shoe-to-drum clearance.
A first aspect of this invention is a drum brake device with an automatic shoe clearance adjustment device, in which, the automatic shoe clearance adjustment device extends between a pair of adjacent ends of facing brake shoes comprising: a strut for restricting return positions of said brake shoes, the strut follows one of the brake shoes in response to an over-movement of the brake shoes and extends automatically an effective length thereof with one part of the strut being away from one of the brake shoes, wherein a shoe clearance over-adjustment prevention apparatus comprises a thermo-sensitive member. The thermo-sensitive member functionally engages with the strut so as to pressurize the same toward the other brake shoe side upon reaching a brake temperature to a predetermined value or range.
A second aspect of this invention is a shoe clearance over-adjustment prevention apparatus for a drum brake as above in which the thermo-sensitive member is a plate member, having superposed sections thereof formed by folding, sandwiched between the strut and the brake shoe.
A third aspect of this invention is a shoe clearance over-adjustment prevention apparatus for a drum brake as above in which the thermo-sensitive member is a plate member, having superposed sections formed by folding, sandwiched between the strut and a brake lever capable of moving the brake shoe.
A fourth aspect of this invention is a shoe clearance over-adjustment prevention apparatus for a drum brake as above in which the thermo-sensitive member has a rotation regulator regulating a play of the strut in the rotational direction relative to the longitudinal direction of the strut.